Military authorities at the Quantico base in northern Virginia have launched an investigation after a US marine shot dead two of his colleagues before killing himself.

On Friday morning, Colonel David Maxwell, the base commander, gave no motive to the attack and said he anticipated a "lengthy" investigation into the incident.

Two marines – a man and a woman – were killed before the gunman shot himself at a barracks that housed applicants for Quantico's officer candidate school. It prompted a lockdown amid fears of a standoff. The base is about 37 miles south of Washington and is also home to the FBI's training academy.

Officers later discovered the body of the gunman in a dorm room after responding to the late-night emergency call. He was pronounced dead at the scene, authorities said, from a self-inflicted gunshot wound. The bodies of two victims were also found at the same barracks.

Maxwell said the shootings were "a tragic loss for our marine corps family".

He told reporters the gunman and both victims were active-duty marines. All three were permanent personnel assigned to the officer candidate school.

Maxwell said: "At 10.30pm, a 911 call was received by the provost marshal's office. Within five minutes of notification, authorities were on the scene. They did not report hearing any shot or firing any shots."

But that account seems to contradict earlier reports of a standoff between the suspect and authorities.

Following the attack, a loud speaker was used to order all other staff and residents on the base to stay indoors. That lockdown notice was lifted at 2.30am after it was confirmed that the attack took place in a single barracks.

The names of the three dead marines have not yet been released. But Maxwell confirmed that the two victims were colleagues of the gunman.

In a statement, Pentagon press secretary George Little said defense secretary Chuck Hagel was "saddened" to hear of the shootings.

"This tragedy, as well as the tragedy in Nevada earlier this week, took the lives of marines who volunteered to serve their nation," Little said.

"[Hagel's] heart and prayers are with them and their families. He believes that the legendary strength of the United States marine corp will ensure that they are forever remembered."